Fix name string representation

string.join expects strings, not StringElement:

```
openprovider.py/openprovider/models.pyc in __str__(self)
   114     def __str__(self):
   115         if hasattr(self, "prefix"):
--> 116             return " ".join((self.first_name, self.prefix, self.last_name))
   117         else:
   118             return " ".join((self.first_name, self.last_name))

TypeError: sequence item 0: expected string, lxml.objectify.StringElement found
```